We have observed the ultrastructure of a canine leukocyte infected with Hepatozoon canis by transmission electron microscope. The infected leukocytes contained many small vesicles and a few typical granules in the cytoplasm. They also contained a few rough surfaced endoplasmic reticula, mitochondria and a segmented nucleus with condensed chromatin. Cytochemical analysis indicated that the infected leukocytes with H. canis contained little alkaline phosphatase, peroxidase, naphthol AS-D chloroacetate esterase or alpha naphthol acetate esterase. These results suggested that the infected leukocytes were neutrophils or monocytes which lost their typical characteristics.